The present invention relates to automatic equalizers which compensate for the distorting effects of bandlimited channels on transmitted data signals.
Automatic equalizers are necessary for accurate reception of high-speed data signals transmitted over bandlimited channels with unknown transmission characteristics. The equalizer, which forms a part of an overall data receiver, is generally in the form of a transversal filter in which successive line samples of a previously-filtered incoming data signal are multiplied by respective tap coefficients. The resulting products are added together to generate an equalizer output which is then demodulated and/or quantized to recover the transmitted data. In addition, an error signal is formed equal to the difference between the equalizer output and a reference signal which represents the transmitted data symbol. The value of the symbol that was transmitted may be known at the receiver a priori, as is the case in many equalizer start-up arrangements. Alternatively, in the so-called adaptive type of automatic equalizer, the reference signal is derived from the decision made in the receiver (on the basis of the equalizer output value) as to what data symbol was transmitted. In either case, the error signal is used to update the tap coefficient values in accordance with an algorithm which minimizes a measure of the distortion--assumed to be primarily intersymbol interference--introduced by the channel.
In some applications, the equalization process entails specialized kinds of signal processing. In particular, as shown in U.S. Pat. No. 3,755,738 issued Aug. 28, 1973, to R. D. Gitlin et al, for example, recovery of the data contained in a quadrature-amplitude-modulated (QAM) signal conventionally involves generation of two versions of the received passband signal--a so-called Hilbert transform pair. These may be generated, for example, by a "phase splitter" comprised of analog filter sections. While generally satisfactory in operation, the analog phase splitter is relatively bulky, must be manufactured to close tolerances, and must be individually tested at the factory. Its parameters are also subject to variation due to such effects as temperature drift and component aging.
The above problems can be ameliorated by realizing the phase splitter with integrated circuit active filters. These, however, are sensitive to radio frequency interference created by static discharges. The phase splitter could also be realized with digital circuitry. The principal drawback to this approach is that it substantially increases the time required to begin forming the data decisions.